Semicircularducts

Vestibular nerve

Cochlear nerve

Vestibulocochlearnerve (VIII)

Internal

auditory meatus

Cochlea

Figure 14.34

The Ear

Pinna

External auditory

meatus

Tympanicmembrane

Malleus

Incus

Stapes

Outer ear

Middle ear

Inner ear

Ear Anatomy & Physiology

Outer Ear(pinna and ear canal): Has air medium

Serves to direct and transmit sound.

TympanyMembrane

Transduces air vibration into mechanical vibration and amplifies the intensity ofthe vibration.

Middle Ear(malleus, Incus, Stapes)

If there is no impedance balancing of the air medium from the outer ear and theliquid medium of the inner ear, only ~0.1% of the incident power of the soundwaves would be transmitted (i.e., an attenuation of 30 dB).

The lever system of theossiclesincreases the vibration by ~1.3 times
The surface area of thetympanymembrane is much larger than that of the stapes,which increases the pressure on the oval window by ~35 times.
Since fluid has greater inertia than air, larger pressure is needed to causevibration in the fluid (impedance matching).

Ear Physiology

Transduction of sound into an auditory perception

Sound is a propagating pressure wave.

Perception of sound involves the electrical activity of neurons in the auditorycortex of the brain.

The transduction process is the means by which the pressure waves in air (amechanical stimulus) is converted into neural activity (action potentials).

This process involves a number of stages, some of which involve conductionand impedance matching.

The path of sound

ear canalvibrate tympanic membrane

vibrateossicles(3 bones: Malleus, Incus, Stapes)

vibrate oval window of cochlea

create waves in cochlea fluid

create standing waves in basilar membrane

movement of hair cells generates electrical activity through mechanicallygated ionic channels

hair cells stimulate the auditory nerve

series of action potentials up to the auditory cortex.

Organ of Corti

Organ ofCorti: The transducer

Vibration of the basilar membrane produces shear forces thatbend the stereocilia (hairs protruding from the hair cells)against the tectorial membrane

Movement of the stereocilia either cause the hair cell todepolarise or hyperpolarise, depending upon the direction ofmovement

Changes in the membrane potential of the hair cell generate an APin the nerve fibre attached to the hair cell.

Electron Micrograph: Apparent are inner hair cell and outer haircell and rows ofstereocillia

Hair Cell Innervation

Inner Hair Cells

–Afferent,
–type I auditory nerve cells
–10-30 AN-I synapses per IHC
–Each AN-I cell has 1 synapse with an IHC
–3500 IHC, 30 000 type I AN cells

Outer Hair Cells

–Efferent
–type II auditory nerve cells
–~6 AN-II synapses per OHC
–Each AN-II cell has ~10 synapses with OHCs
–~11 000 OHC, 1800 type II AN cells

Resting Electrical Potentials of the Cochlear

Two classes of potential: Resting potentials and stimulus-related potentials

Maintenance of large DC potentials between endolymphatic andperilymphaticspaces plays an important role in active cochlear tuning

Endolymph:

Located inscalamedia
Composition similar to intracellular fluid: High K+, low Na+,
Endocochlearpotential(EP): restingd.c.potential of scale media is approx.+80mV (relative toscalatympani)
EP provides the electrical driving force required for excitation of hair cells(the cochlear battery–turning the EP off silences the cochlear)
EP maintained by metabolically active processes

Perilymph:

Located in bothscalatympani andscalavestibuli
Composition similar to extracellular fluid and CSF: Low K+, high Na+,
Electrical potential is approx. zero (+/-5mV)

Active Cochlea Mechanics

Accounts for extraordinarily sharp tuning of auditory nerve fibres
Post mortem studies show rapid deterioration of tuning as the physiologicalcondition deteriorates (only a broad tuning remains)
Outer hair cells are triggered by the travelling wave to feed mechanical energyback into the BM (i.e., active process), which significantly enhances the tuningproperties of the BM
The active process is a positive feedback mechanism
An alternating electrical field causes the outer hair cells to shorten and lengthen
The active process explains the origin of cochlear emissions

Frequency Tuning Curve

log frequency

100

Characteristic

Frequency

Threshold

Bandwidth

log amplitude (dB SPL)

The discharges of cochlear nerve fibres to low-frequency sounds arenot random;

they occur at particular times (phase locking).

Evans (1975)

Temporal Frequency Coding

Temporal Coding

Information is contained in the relative timing of individual action potentials.E.g.

Frequency is encoded in the inverse of the minimum inter-spike intervals.
Time of arrival of signal at ear.

The temporal code contains detailed information about the phase of a periodicsignal, i.e. much more fine-grained information about the stimulus down to aresolution of ~10 us at a population level.

Note: This information is in addition to rate information, not in contradiction ofit.

Temporal information is observed in the auditory pathway, but diminishes inresolution from auditory nerve to auditory cortex (~10 us to ~10 ms)

Does temporal information play a functional role or is it an epiphenomenon?

What evidence is there that the temporal code is used by the brain?

Summary

Outer Ear: amplification and direction filtering.

Middle Ear: impedance matching.

Cochlear & Basilar Membrane: spectral analyser.

Organ ofCorti: Acoustic to neuro-electric transducer.

Inner Hair Cells: Synapse type I AN fibres projectingcentrally.

Outer Hair Cells: Amplification and improved frequencytuning.

Auditory Nerve: Carries signal to the auditory braincentres. Information is convey by both rate and timing ofaction potentials.

Octopus

Spherical bushy

Globular bushy

Fusiform

Stellate

Ventral

Posterior

Dorsal

DCN

AVCN

PVCN

Cochlear Nucleus

Parallel pathways emerge from the CN

VCN extracts and enhances timing and spectrum, as well as information on thewaveform envelope (e.g., amplitude modulation)

Bushy cellsTBSOCDNLLIClocalisation“where”

T-stellate cellTBVNLLICspectrum, AM“what”

Octopus cellVNLLICtemporal pattern

DCNIClocalisation & spectrum

monaural pinna (spectral) cues

These parallel streams converge in the central nucleus of the inferior colliculus (IC)in the midbrain

Abbreviations:

TB–Trapezoid body

SOC–superior olivary complex

DNLL–dorsal nucleus of the lateral lemniscus

VNLL–ventral nucleus of the lateral lemniscus

IC–interior colliculus

Auditory Cortex

from http://www.physiology.wisc.edu/neuro524/audition02-524.htm

Left hemisphere is specialised in the analysis, perception and production oflanguage.

Right hemisphere implicated in the perception of duration, intensity,intonation, melody.

Laminar Structure

From: Fundamental Neuroscience (2002), Elsevier, Fig 8, CH 22

Supragranular

layers (1-3)

Granular layer (4)

Infragranular

layers (5-6)

stellate cell

Structure of Cerebral Cortex

Cerebral cortex consists ofsix layers, numbered from the surface. Each layer has a distinctpattern of inputs and/or outputs and cell type..
Layer 4 is the input layer. In contrast to other areas (visual, somatosensory) stellate cells are notthe primary recipient of thalamic input. Receives input from thalamus.
Layers 2/3 are output layers. Projections to auditory association cortices, by excitatorypyramidal cells only
Layers 5 & 6 feedback projections: to subcortical nuclei such as the thalamus or inferiorcolliculus.

Columnar Organisation

Both anatomical and electrophysiological evidence supports acolumnar organization of auditory cortex.

Neurons in vertical electrode penetrations have similar responseproperties. Eg. CF, binaural response, bandwidth.

Synaptic connections between neurons with similar responseproperties

Cortical Maps

Spatial clustered or topographic arrangement of neural responseacross the cortical surface.

Cortical maps of:

Frequency (tonotopic)

Binaural response type

Bandwidth.

Binaural and bandwidth maps are roughly orthogonal to tonotopicmap.

Summary of the Auditory Pathway

Cochlear nucleus: parallel pathways–what, where, temporal andrate coding
Superior Olivary Complex: sound localisation
Lateral Lemniscus: sound localisation
Inferior Colliculus (midbrain): integration
Medial Geniculate Body (thalamus)
Auditory Cortex–higher order auditory features: pitch, timbreetc

Tonotopic organisation

BMEN20002Anatomy &Physiology forBioengineering

Overview

The Ear

Basilar membrane travelling wave

Cochlear duct

Cochlear Duct

Organ ofCorti

Organ of Corti

Hair Cell Innervation

Hair Cell Innervation

Receptor Potential

Auditory Nerve

Frequency Tuning Curve

Summary

The Auditory Pathway

Brainstem & Midbrain

Brainstem & Thalamus

Columnar Organisation

Columnar Organisation

Cortical Maps

Cortical Maps